1. Field of the Invention
This invention relates to electrolumanescent (EL) lamps and, more specifically, to a dimmable electrolumanescent lamp driver.
2. Description of the Prior Art
Electrolumanescent (EL) lamps are basically capacitors that contain a phosphor so that a light is given off when a displacement current flows. Because of this, the light output of a particular lamp is to a first order directly dependent on the product of the operating voltage and frequency, and to a lesser extent on the waveshape being used to drive the lamp. As a matter of convenience, most integrated circuits generate an approximately square waveshape for driving the two lamp electrodes with a constant peak-to-peak voltage. The brightness of an EL lamp is greater if the square wave voltage applied to the lamp has a finite rise and fall time of 5% or more of the operating wave period. Higher efficiency operation is obtained if during the wave transition from one polarity to the opposite, the lamp terminals are shorted to each other first to partially discharge the lamp capacitance. Otherwise, the current required for the discharge comes from the high voltage power source and causes wasted energy.
Control of the EL lamp's brightness may be implemented by providing a mechanism to vary the lamp drive voltage or the lamp operation frequency. However, circuits to control the brightness of an EL lamp have many drawbacks. For example, brightness control by drive frequency is limited in the minimum brightness which it can produce. Furthermore, many EL lamp drivers do not allow controlling the brightness of multiple EL lamps.
Therefore, a need existed to provide an improved EL lamp driver. The improved EL lamp driver must be able to overcome the problems associated with the prior art EL lamp drivers.